Electric linear motion actuator and electric brake system

ABSTRACT

In an electric linear motion actuator for transmitting torque of a rotary shaft ( 4 ) to planetary rollers ( 7 ) and thus to a linearly driven output member constituted by an outer race member ( 5 ), axial movement of a carrier ( 6 ) supporting the planetary rollers ( 7 ) is restricted, and the outer race member ( 5 ) is axially slidably fitted in a radially inner surface of a cylindrical portion ( 1   a ) of a housing ( 1 ) and is rotationally fixedly coupled to an object to be driven via keys ( 25 ) The planetary rollers ( 7 ) have their radially inner surfaces rotatably supported on respective support pins ( 6   c ) of the carrier ( 6 ), and are radially inwardly biased by radial compression ring springs ( 21 ) wound around the support pins ( 6   c ) so as to envelop the support pins. The planetary rollers ( 7 ) are thus pressed against the radially outer surface of the rotary shaft ( 4 ).

TECHNICAL FIELD

The present invention relates to an electric linear motion actuator for converting the rotary motion of an electric motor to a linear motion of a object to be driven, and an electric brake system including a member to be braked and a braking member which is adapted to be pressed against the member to be braked by the electric linear motion actuator.

BACKGROUND ART

Many electric linear motion actuators for converting the rotary motion of an electric motor to a linear motion of an object to be driven include a ball-screw mechanism or a ball-ramp mechanism as a motion converter means. Further, many of such linear motion actuators include a gear reduction mechanism such as a planetary gear speed reducer so as to obtain a large linear driving force with a small-capacity electric motor (see e.g. JP Patent Publication 6-327190A).

The ball-screw mechanism or ball-ramp mechanism used in the above-described electric linear motion actuators has the ability to increase the driving force to some extent because it includes threads having a lead angle or inclined cam surfaces. But this motion converting mechanism alone cannot sufficiently increase the driving force as required by e.g. an electric brake system. Thus, the electric linear motion actuators using this type of motion converter mechanism further include a separate speed reducer such as a planetary gear speed reducer to increase the driving force. But the addition of such a separate speed reducer increases the size of the entire electric linear motion actuator.

The inventors of this application proposed an electric linear motion actuator which is free of this problem, which can sufficiently increase the driving force without mounting a separate speed reducer, and which can be used in an electric brake system, of which the linear motion stroke is relatively short. This actuator comprises a rotary shaft to which the rotation of a rotor shaft of an electric motor is configured to be transmitted, a housing having a radially inner surface, an outer race member fitted in the radially inner surface of the housing and provided around the rotary shaft, a carrier, and a plurality of planetary rollers disposed between the outer race member and the rotary shaft and rotatably supported by the carrier, wherein the planetary rollers are configured to revolve around the rotary shaft while rotating about axes of the respective planetary rollers when the rotary shaft rotates, wherein a helical rib is formed on the radially outer surface of the rotary shaft or on the radially inner surface of the outer race member, and wherein each of the planetary rollers has, on a radially outer surface thereof, a plurality of circumferential grooves which are arranged at the same pitch as the helical rib and in which the helical rib is engaged, or a helical groove in which the helical rib is engaged, the helical groove having a different lead angle from the helical rib and being arranged at the same pitch as the helical rib, thereby axially moving the carrier. Thus, the rotation of the rotary shaft is converted to a linear motion of the carrier. In this arrangement, the carrier or a linear motion member coupled to the carrier serves as an output member for linearly driving an object to be driven (JP Patent Publication 2007-32717A and JP Patent Publication 2007-37305A).

On the other hand, while many vehicle brake systems are hydraulic ones, with the recent introduction of sophisticated brake control such as anti-lock brake control system (ABS), electric brake systems are gathering attention because electric brake systems can perform such sophisticated brake control without the need for complicated hydraulic circuits. In an electric brake system, an electric motor is activated in response to a signal produced when the brake pedal is depressed, thereby pressing the braking member against the member to be braked through the above-described electric linear motion actuator, which is mounted in the caliper body (see e.g. JP Patent Publication 2003-343620A).

The electric linear motion actuator disclosed in JP Patent Publication 2007-32717A or JP Patent Publication 2007-37305A, in which the carrier or the linear motion member coupled to the carrier serves as an output member which makes a linear motion, can sufficiently increase the driving force without the need to mount a separate speed reducer and thus is compact in size. But since the carrier or the linear motion member, which undergoes a linear motion, is relatively short in the axial direction, if this electric linear motion actuator is used e.g. in an electric brake system, a tangential force applied from the member to be braked to the braking member as the object to be driven partially acts on the carrier or the linear motion member as a lateral moment. This lateral moment may hamper smooth linear motion of the carrier or the linear motion member. If a linear motion member guided by the outer race member is coupled to the brake member, as the braking member becomes worn, the axial length of the portion of the linear motion protruding from the outer race member increases while the axial length of its portion guided by the outer race member correspondingly decreases. This makes smooth linear motion of the linear motion member even more difficult.

Also, in order to stably transmit rotation of the rotary shaft as the input shaft to the respective planetary rollers, the planetary rollers are disposed between the rotary shaft and the outer race member with negative gaps, thereby applying a preload to the respective planetary rollers between the radially outer surface of the rotary shaft and the radially inner surface of the outer race member. This makes it necessary to mount the planetary rollers between the outer race member and the rotary shaft by e.g. interference fit. Also, in order to control the negative gaps for applying the preload, it is necessary to finish the radially outer surface of the rotary shaft and the radially inner surface of the outer race member with high accuracy by e.g. grinding. It is thus troublesome and time-consuming to finish the rotary shaft and outer race member and then mount the planetary rollers. This increases the manufacturing cost.

SUMMARY OF THE INVENTION

An object of the present invention is to allow smooth guidance of the linear motion of the output member even when a lateral moment acts on the output member, and to make it possible to stably transmit torque of the rotary shaft to the respective planetary rollers without applying a preload to the planetary rollers by negative gaps between the rollers and the radially outer surface of the rotary shaft and between the rollers and the radially inner surface of the outer race member.

In order to achieve this object, the present invention provides an electric linear motion actuator comprising a rotary shaft to which the rotation of a rotor shaft of an electric motor is to be transmitted, a housing having a radially inner surface, an outer race member fitted in the radially inner surface of the housing and provided around the rotary shaft, a carrier having a plurality of support pins, and a plurality of planetary rollers disposed between the outer race member and the rotary shaft and rotatably supported by the respective support pins of the carrier, wherein the planetary rollers are configured to revolve around the rotary shaft while rotating about axes of the respective planetary rollers when the rotary shaft rotates, wherein a helical rib is formed on a radially inner surface of the outer race member, and wherein each of the planetary rollers has, on a radially outer surface thereof, a plurality of circumferential grooves which are arranged at the same pitch as the helical rib and in which the helical rib is engaged, or a helical groove in which the helical rib is engaged, the helical groove having a different lead angle from the helical rib and being arranged at the same pitch as the helical rib, thereby axially moving the outer race member and the carrier relative to each other, whereby the rotary motion of the rotary shaft is converted to a linear motion of an output member, thereby driving an object to be driven coupled to the output member, wherein the axial movement of the carrier is restricted, and the outer race member is rotationally fixed and is axially slidably fitted in the radially inner surface of the housing, whereby the outer race member serves as the output member, and that the actuator further comprises an elastic member biasing the planetary rollers against a radially outer surface of the rotary shaft.

In this arrangement, since the carrier is axially immovable, and the outer race member is fitted in the housing so as to be non-rotatable but axially movable so that the outer race member serves as the output member, it is possible to guide the outer race member as the output member over its axially long area with the radially inner surface of the housing. This in turn makes it possible to smoothly guide the linear motion of the output member even when a lateral moment acts on the output member. By providing the elastic member biasing the planetary rollers against a radially outer surface of the rotary shaft, it is possible to stably transmit torque of the rotary shaft to the respective planetary rollers without applying a preload to the planetary rollers by negative gaps between the rollers and the radially outer surface of the rotary shaft and between the rollers and the radially inner surface of the outer race member.

As means for biasing the planetary rollers, the support pins may be mounted to the carrier so as to be circumferentially immovable and radially movable, and rotatably support the respective planetary rollers at a radially inner surface of each planetary roller, and the elastic member biases the respective support pins radially inwardly toward the rotary shaft.

The elastic member may be a ring-shaped elastic member enveloping the support pins so as to tend to be radially compressed.

The ring-shaped elastic member may be a radial compression spring made of spring steel and having circumferentially spaced apart ends.

Preferably, the rotary shaft is supported by a single bearing at one axial position of the rotary shaft, and the rotary shaft is indirectly supported by an annular member having an annular portion configured to be brought into contact with the radially inner surface of the outer race member when the annular portion minutely moves in a radial direction at an axial position of the rotary shaft axially spaced from the axial position of the rotary shaft where the rotary shaft is supported by the single bearing. With this arrangement, even when the planetary rollers are pressed against the radially outer surface of the rotary shaft, no moment loads acts on the bearing for the rotary shaft.

The rotary shaft may be indirectly supported by the annular member at a plurality of axial positions of the rotary shaft which are axially spaced apart from each other and from the axial position of the rotary shaft where the rotary shaft is supported by the single bearing so as to more reliably prevent moment loads from acting on the bearing for the rotary shaft.

Preferably, the annular member is brought into contact with the radially inner surface of the outer race member through a slide bearing so as to reduce friction loss when the rotary shaft rotates.

The slide bearing is preferably press-fitted on the annular member.

The present invention also provides an electric brake system comprising a member to be braked, a braking member, and an electric linear motion actuator for converting a rotary motion of an electric motor to a linear motion of the braking member, thereby pressing the braking member against the member to be braked, wherein the electric linear motion actuator is of the above-described type. With this arrangement, even when a tangential force applied from the braked member to the braking member acts on the output member as lateral moment, it is possible to smoothly guide the linear motion of the output member.

According to the present invention, since the carrier is axially immovable, the outer race member is fitted in the housing so as to be non-rotatable but axially movable so that the outer race member serves as the output member, and the elastic member is provided that presses the planetary rollers against the radially outer surface of the rotary shaft, it is possible to smoothly guide the linear motion of the output member even when a lateral moment acts on the output member, and to stably transmit torque of the rotary shaft to the respective planetary rollers without applying a preload to the planetary rollers by negative gaps between the rollers and the radially outer surface of the rotary shaft and between the rollers and the radially inner surface of the outer race member.

In the electric brake system according to the present invention, the braking member to be pressed against the braked member is driven linearly using the above-mentioned linear motion actuator. Thus, even when a tangential force applied from the braked member to the braking member acts on the output member as a lateral moment, it is possible to smoothly guide the linear motion of the output member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of an electric linear motion actuator embodying the present invention.

FIG. 2 is a sectional view taken along line II-II of FIG. 1.

FIG. 3 is a sectional view taken along line III-III of FIG. 1.

FIG. 4 is a side view of a ring-shaped radial compression spring shown in FIG. 1.

FIGS. 5( a) and 5(b) are front views showing a helical rib on the outer race member of FIG. 1 and a helical groove on a planetary roller of FIG. 1.

FIG. 6 is a vertical sectional view of an electric brake system using the electric linear motion actuator of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Now the embodiment of the invention is described with reference to the drawings. As shown in FIGS. 1 to 3, the electric linear motion actuator of the embodiment includes a housing 1 having a cylindrical portion 1 a and a flange 1 b extending to one side from one end of the cylindrical portion 1 a. An electric motor 2 is mounted to the flange 1 b so as to be parallel to the cylindrical portion 1 a. The electric motor 2 has a rotor shaft 2 a of which the rotation is transmitted to a rotary shaft 4 extending along the axis of the cylindrical portion 1 a through gears 3 a, 3 b and 3 c. Four planetary rollers 7 are disposed between the rotary shaft 4 and an outer race member 5 axially slidably fitted in the radially inner surface of the cylindrical portion 1 a. The planetary rollers 7 are rotatably supported by a carrier 6, and adapted to revolve around the rotary shaft 4 while rotating about their own axes when the rotary shaft 4 rotates.

A lid 1 c is mounted to the housing 1 at its end where the flange 1 b is provided. The gears 3 a, 3 b and 3 c mesh with each other in the same plane in the closed space defined by the lid 1 c. A shaft support member 8 is mounted in the cylindrical portion 1 a at its end portion where the lid 1 c is mounted. The rotary shaft 4 has its proximal end portion, where the gear 3 c is mounted, supported by the shaft support member 8 through a ball bearing 9. The shaft support member 8 is fixed to the housing 1 by means of snap rings 10 a and 10 b, and serves to restrict axial movements of the rotary shaft 4 and the carrier 6 too. The intermediate gear 3 b, which meshes with the gear 3 a, which is mounted to the rotor shaft 2 a, and the gear 3 c is supported by a shaft pin 11 extending between the flange 1 b and the lid 1 c through a ball bearing 12.

The carrier 6 comprises a carrier body 6 a and a support plate 6 b which are spaced from each other and rotatably fitted around the rotary shaft 4 through slide bearings 13 a and 13 b, respectively, support pins 6 c having their ends supported by the carrier body 6 a and the support plate 6 b, respectively, and rotatably supporting the respective planetary rollers 7, and a plurality of coupling rods 6 d through which the support plate 6 b and the carrier body 6 a are coupled together so as to be angularly aligned with each other. Each coupling rod 6 d has its ends coupled to the carrier body 6 a and the support plate 6 b by bolts 6 e, respectively.

The carrier body 6 a is supported on the end surface of the shaft support member 8 by means of a thrust roller bearing 14 through a support member 6 f so as to be rotatable about the rotary shaft together with the planetary rollers 7. The shaft support member 8 thus restricts the movement of the carrier body 6 a toward the proximal end of the rotary shaft 4. A snap ring 15 fitted on the rotary shaft 4 at its distal end prevents separation of the support plate 6 b, which is coupled to the carrier body 6 a through the coupling rods 6 d, through a slide bearing 16 and restricts its movement toward the distal end of the rotary shaft 4. Thus, the movement of the carrier 6 toward either end of the rotary shaft 4 is restricted.

The planetary rollers 7 are rotatably supported on the respective support pins 6 c of the carrier 6 through needle roller bearings 17 and are supported on the carrier body 6 a through thrust roller bearings 18 so as to be rotatable about their own axes. Each support pin 6 c has its ends received in radially elongated holes 19 formed in the carrier body 6 a and the support plate 6 b, respectively, so as to be circumferentially immovable and radially movable.

Grooves 20 are formed on the radially outer surface of each support pin 6 c at the respective end portions thereof. Radial compression ring springs 21 made of spring steel and having circumferentially spaced apart ends as shown in FIG. 4 are fitted in the grooves 20 at the respective ends of the support pins 6 c in a radially expanded state so as to envelop the support pins 6 c. The planetary rollers 7, which are rotatably supported by the respective support pins 6 c, are thus pressed against the radially outer surface of the rotary shaft 4 by the springs 21 but, as shown in FIG. 1, the planetary rollers 7 are not in threaded engagement with the rotary shaft 4. This allows stable transmission of torque of the rotary shaft 4 to the respective planetary rollers 7.

The coupling rods 6 d are disposed between the respective adjacent planetary rollers 7 and couple the carrier body 6 a to the support plate 6 b. A fan-shaped lubricant applicator member 22 is held in position between each coupling rod 6 d and the radially inner surface of the outer race member 5 which is in sliding contact with the radially outer surfaces of the planetary rollers 7 on both circumferential sides to apply grease thereto. A seal member 23 is fitted on the radially inner surface of the outer race member 5 for isolating the interior of the outer race member, in which the planetary rollers 7 and the lubricant applicator members 22 are mounted, from the outside. The seal member 23 is made by pressing a thin steel sheet and includes a cylindrical outer portion fitted in the outer race member 5.

The carrier body 6 a and the support plate 6 b are annular members adapted to be brought into contact with the radially inner surface of the outer race member 5 through slide bearings 24 a and 24 b when minutely moved in a radial direction. The carrier body 6 a and the support plate 6 b thus indirectly support the rotary shaft 4, thereby preventing moment loads from acting on the ball bearing 9, which supports the rotary shaft 4. The slide bearings 24 a and 24 b are press-fitted and fixed on the radially outer surfaces of the carrier body 6 a and the support plate 6 b, respectively.

An object to be driven is coupled to the distal end of the outer race member 5. The object to be driven is rotationally fixed to the outer race member by means of keys 25 formed on the distal end surface of the outer race member. Thus, the outer race member 5, which is axially slidably fitted in the radially inner surface of the cylindrical portion 1 a of the housing 1, is axially movable relative to the carrier 6, of which the movements in both axial directions are restricted relative to the housing 1, and thus serves as an output member. The end of the cylindrical portion 1 a to which the object to be driven is coupled is open. Thus the outer race member 5 can undergo a linear motion with a long stroke because it can protrude from the cylindrical portion 1 a. Even when the outer race member 5 protrudes from the cylindrical portion, it is guided by the radially inner surface of the cylindrical portion 1 a over an axially long area, so that the outer race member can smoothly undergo a linear motion.

As shown in FIG. 5( a), two helical grooves 5 a are formed on the radially inner surface of the outer race member 5, with which the planetary rollers 7 are brought into rolling contact. Separate rib members 5 b are fixedly fitted in the respective helical grooves 5 a, forming two helical ribs on the radially inner surface of the outer race member 5. As shown in FIG. 5( b), a single helical groove 7 a is formed on the radially outer surface of each planetary roller 7, which is the same in pitch as and different in lead angle from the helical ribs formed by the rib members 5 b, and in which the helical ribs are engaged. Due to the difference in lead angle between the helical ribs and the helical grooves 7 a, when the rotary shaft 4 rotates and the planetary rollers 7 revolve around the rotary shaft 4 while rotating about their own axes, with the helical ribs engaged in the helical grooves 7 a, the outer race member 5 moves axially relative to the planetary rollers 7. In the embodiment, two helical ribs are formed on the outer race member so that the difference in lead angle between the helical ribs and the helical grooves 7 a of the planetary rollers 7 can be determined more freely. But a single helical rib may be formed instead. Instead of the helical groove 7 a, a plurality of circumferential grooves may be formed on each planetary roller 7 a at the same pitch as the helical ribs.

FIG. 6 shows an electric brake system in which the above-described electric linear motion actuator is used. This electric brake system is a disk brake including a caliper body 31, a disk rotor 32 as a member to be braked, and brake pads 33 as braking members provided on both sides of the disk rotor 32 in the caliper body 31 so as to face each other. The housing 1 of the electric linear motion actuator is fixed to the caliper body 31, and its outer race member 5 as the output member is rotationally fixed to one of the brake pads 33 as the object to be braked through the keys 24 so that the brake pads 33 can be pressed against the disk rotor 32. In FIG. 6, the electric linear motion actuator is shown in section taken along a plane perpendicular to the plane containing the section of FIG. 1.

In the above embodiment, the helical ribs on the radially inner surface of the outer race member are formed by separate rib members fitted in the helical grooves. But instead, the helical ribs may be formed integral with the outer race member. 

What is claimed is:
 1. An electric linear motion actuator comprising: a rotary shaft to which the rotation of a rotor shaft of an electric motor is to be transmitted; a housing having a radially inner surface; an outer race member fitted in the radially inner surface of the housing and provided around the rotary shaft; a carrier having a plurality of support pins; a plurality of planetary rollers disposed between the outer race member and the rotary shaft and rotatably supported by the respective support pins of the carrier; and an elastic member biasing the planetary rollers against a radially outer surface of the rotary shaft; wherein the planetary rollers are configured to revolve around the rotary shaft while rotating about axes of the respective planetary rollers when the rotary shaft rotates; wherein a helical rib is formed on a radially inner surface of the outer race member, and wherein each of the planetary rollers has, on a radially outer surface thereof, a plurality of circumferential grooves which are arranged at the same pitch as the helical rib and in which the helical rib is engaged, or a helical groove in which the helical rib is engaged, said helical groove having a different lead angle from the helical rib and being arranged at the same pitch as the helical rib, thereby axially moving the outer race member and the carrier relative to each other, whereby the rotary motion of the rotary shaft is converted to a linear motion of an output member, thereby driving an object to be driven coupled to the output member; wherein said planetary rollers are not in threaded engagement with said rotary shaft; and wherein the axial movement of the carrier is restricted, and the outer race member is rotationally fixed and is axially slidably fitted in the radially inner surface of the housing, whereby the outer race member serves as the output member.
 2. The electric linear motion actuator of claim 1, wherein the support pins are mounted to the carrier so as to be circumferentially immovable and radially movable, and rotatably support the respective planetary rollers at a radially inner surface of each planetary roller, and wherein said elastic member biases the respective support pins radially inwardly toward the rotary shaft.
 3. The electric linear motion actuator of claim 2, wherein the elastic member is a ring-shaped elastic member enveloping the support pins so as to tend to be radially compressed.
 4. The electric linear motion actuator of claim 3, wherein the ring-shaped elastic member is a radial compression spring made of spring steel and having circumferentially spaced apart ends.
 5. The electric linear motion actuator of claim 4, wherein the rotary shaft is supported by a single bearing at one axial position of the rotary shaft, and wherein the rotary shaft is indirectly supported by an annular member having an annular portion configured to be brought into contact with the radially inner surface of the outer race member when the annular portion minutely moves in a radial direction at an axial position of the rotary shaft axially spaced from the axial position of the rotary shaft where the rotary shaft is supported by the single bearing.
 6. An electric brake system comprising a member to be braked, a braking member, and an electric linear motion actuator for converting a rotary motion of an electric motor to a linear motion of the braking member, thereby pressing the braking member against the member to be braked, wherein the electric linear motion actuator is the electric linear motion actuator of claim
 4. 7. An electric brake system comprising a member to be braked, a braking member, and an electric linear motion actuator for converting a rotary motion of an electric motor to a linear motion of the braking member, thereby pressing the braking member against the member to be braked, wherein the electric linear motion actuator is the electric linear motion actuator of claim
 2. 8. An electric brake system comprising a member to be braked, a braking member, and an electric linear motion actuator for converting a rotary motion of an electric motor to a linear motion of the braking member, thereby pressing the braking member against the member to be braked, wherein the electric linear motion actuator is the electric linear motion actuator of claim
 3. 9. An electric brake system comprising a member to be braked, a braking member, and an electric linear motion actuator for converting a rotary motion of an electric motor to a linear motion of the braking member, thereby pressing the braking member against the member to be braked, wherein the electric linear motion actuator is the electric linear motion actuator of claim
 1. 10. The electric linear motion actuator of claim 2, wherein the rotary shaft is supported by a single bearing at one axial position of the rotary shaft, and wherein the rotary shaft is indirectly supported by an annular member having an annular portion configured to be brought into contact with the radially inner surface of the outer race member when the annular portion minutely moves in a radial direction at an axial position of the rotary shaft axially spaced from the axial position of the rotary shaft where the rotary shaft is supported by the single bearing.
 11. The electric linear motion actuator of claim 3, wherein the rotary shaft is supported by a single bearing at one axial position of the rotary shaft, and wherein the rotary shaft is indirectly supported by an annular member having an annular portion configured to be brought into contact with the radially inner surface of the outer race member when the annular portion minutely moves in a radial direction at an axial position of the rotary shaft axially spaced from the axial position of the rotary shaft where the rotary shaft is supported by the single bearing.
 12. The electric linear motion actuator of claim 1, wherein the carrier further includes a carrier body rotatably fitted around the rotary shaft, and a carrier plate rotatably fitted around the rotary shaft and axially spaced apart from said carrier body; each of said support pins has a first end supported by the carrier body and a second end supported by the carrier plate; and said planetary rollers are rotatably supported by the support pins, respectively, such that the planetary rollers are disposed axially between the carrier body and the carrier plate.
 13. An electric linear motion actuator comprising: a rotary shaft to which the rotation of a rotor shaft of an electric motor is to be transmitted; a housing having a radially inner surface; an outer race member fitted in the radially inner surface of the housing and provided around the rotary shaft; a carrier having a plurality of support pins; a plurality of planetary rollers disposed between the outer race member and the rotary shaft and rotatably supported by the respective support pins of the carrier; and an elastic member biasing the planetary rollers against a radially outer surface of the rotary shaft; wherein the planetary rollers are configured to revolve around the rotary shaft while rotating about axes of the respective planetary rollers when the rotary shaft rotates; wherein a helical rib is formed on a radially inner surface of the outer race member, and wherein each of the planetary rollers has, on a radially outer surface thereof, a plurality of circumferential grooves which are arranged at the same pitch as the helical rib and in which the helical rib is engaged, or a helical groove in which the helical rib is engaged, said helical groove having a different lead angle from the helical rib and being arranged at the same pitch as the helical rib, thereby axially moving the outer race member and the carrier relative to each other, whereby the rotary motion of the rotary shaft is converted to a linear motion of an output member, thereby driving an object to be driven coupled to the output member; wherein the axial movement of the carrier is restricted, and the outer race member is rotationally fixed and is axially slidably fitted in the radially inner surface of the housing, whereby the outer race member serves as the output member; and wherein the rotary shaft is supported by a single bearing at one axial position of the rotary shaft, and wherein the rotary shaft is indirectly supported by an annular member having an annular portion configured to be brought into contact with the radially inner surface of the outer race member when the annular portion minutely moves in a radial direction at an axial position of the rotary shaft axially spaced from the axial position of the rotary shaft where the rotary shaft is supported by the single bearing.
 14. The electric linear motion actuator of claim 13, wherein the rotary shaft is indirectly supported by the annular member at a plurality of axial positions of the rotary shaft which are axially spaced apart from each other and from the axial position of the rotary shaft where the rotary shaft is supported by the single bearing.
 15. The electric linear motion actuator of claim 14, wherein the annular member is brought into contact with the radially inner surface of the outer race member through a slide bearing.
 16. An electric brake system comprising a member to be braked, a braking member, and an electric linear motion actuator for converting a rotary motion of an electric motor to a linear motion of the braking member, thereby pressing the braking member against the member to be braked, wherein the electric linear motion actuator is the electric linear motion actuator of claim
 14. 17. The electric linear motion actuator of claim 13, wherein the annular member is brought into contact with the radially inner surface of the outer race member through a slide bearing.
 18. The electric linear motion actuator of claim 17 wherein the slide bearing is press-fitted on the annular member.
 19. An electric brake system comprising a member to be braked, a braking member, and an electric linear motion actuator for converting a rotary motion of an electric motor to a linear motion of the braking member, thereby pressing the braking member against the member to be braked, wherein the electric linear motion actuator is the electric linear motion actuator of claim
 17. 20. An electric brake system comprising a member to be braked, a braking member, and an electric linear motion actuator for converting a rotary motion of an electric motor to a linear motion of the braking member, thereby pressing the braking member against the member to be braked, wherein the electric linear motion actuator is the electric linear motion actuator of claim
 18. 21. An electric brake system comprising a member to be braked, a braking member, and an electric linear motion actuator for converting a rotary motion of an electric motor to a linear motion of the braking member, thereby pressing the braking member against the member to be braked, wherein the electric linear motion actuator is the electric linear motion actuator of claim
 13. 